Serum amyloid A (SAA) protein-interaction with itself and serum albumin

Serum amyloid A (SAA) protein is a 12,000 dalton protein that exists in serum under physiologic conditions as an 85,000 dalton complex and under certain conditions, as a 170,000 dalton component. To study the reason for this finding, the behavior of 125I-SAA was studied in the presence of cold SAA and several serum proteins. SAA caused a shift of some of the radioactivity to the region of albumin. Addition of normal human serum or albumin caused a shift of a significant fraction of the radioactivity to a peak eluting slightly ahead of albumin (80.000 daltons). This interaction could be blocked by the addition of cold SAA. No shift was noted when IgG or Bence Jones proteins were added. Thus, it appears that low molecular SAA protein has a tendency to aggregate with itself and to bind to albumin but not to human IgG or Bence Jones proteins.